Lighting fixtures listed by Underwriters' laboratories (UL) as suitable for use in hazardous locations (Class I, Division 1) are required to have enclosures for the electrical components having sufficient strength to withstand the explosion pressure should there be an electrical or other malfunction that ignites the gases inside. Furthermore, as the momentary pressure from the explosion inside relieves itself to the outside, the ignited gas must be cooled sufficiently as they exit that explosive gases on the outside are not ignited. This latter quality is commonly referred to as non flame-propagation through the joints.
External propagation of flame through joints of the enclosure, such as metal-to-metal or metal-to-glass joints, is prevented by limiting the clearance between parts inversely with the length of the path through the joint. Two types of joint are commonly used. One type consists of flat mating or matching rabbeted surfaces and they must meet specified path length and clearance requirements. The other type to which this invention particularly addresses itself utilizes screw threads having a required clearance between mating threads and a specified minimum number of full threads in engagement.
With respect to thread joints, in order to assure consistent clearance between the threads, it has been necessary up to the present time to avoid painting the thread surfaces. Avoidance of paint on thread joints has in fact been mandated by UL on the grounds that if paint were allowed, variations in coverage thickness, type of paint, bubbles etc. could alter the flow characteristics and cooling effects on the explosive exhaust gases. These same variations could also cause difficulty in screwing the mating parts together. Lack of paint on threads favors corrosion, binding in threaded joints, and galling of the thread surfaces particularly where both parts are made of aluminum. In lighting fixtures used in areas with serious corrosion problems, threads not protected by paint or other corrosion inhibitor are vulnerable to rapid deterioration causing enlargement of the clearance path and increasing the probability of the occurrence of flame propagation within the lifetime of the product.
Corrosion and binding in threads may be reduced to some extent by coating the threads with an approved lubricant such as mineral oil base greases, petroleum jelly, or silicone base greases. Approval for the use of lubricant on the screw threads is covered in UL Bulletin of Research Number 4 titled "Effect of Grease in Metal Joints on Safe Operation of Explosion-Proof Electrical Equipment" (Fifth Printing--April 1977). However, according to the bulletin, regardless of the amount of grease applied, only a thin film remains on each joint surface after an explosion test, and deposits of grease in proportion to the amount applied are found on screens placed a few inches away from the joints. Thus while greases of suitable composition do not reduce the margin of safety with respect to non flame-propagation through joints, the greases are blown out whenever an explosion occurs and the protection against corrosion is at best limited and only temporary.